This application claims the benefit under 35 USC 119 of European Patent Application No. 08005381.2, filed Mar. 20, 2008, the disclosure of which is incorporated herein by reference.
Sometimes it is necessary in the field of high pressure hydraulic to control a hydraulic consumer to carry out extremely small movements only. In many cases, this hydraulic consumer is also controlled in a conventional fashion to carry out large movements, and if necessary, even independent of the current load. The mentioned small movements of the hydraulic consumer can be realised only with dedicated structures because of the conventional high working pressures of about 100 to 800 bar, e.g. by means of relatively large dimensioned solenoid valves or throttling valves which then are controlled to open for a very short time and/or very little. This short time or small opening degree needs powerful solenoids and a complicated control device. Moreover, a lot of solenoid actuating primary energy is wasted, and the hydraulic medium is mechanically highly loaded. Small movements of a hydraulic consumer, e.g. only some millimetres, until now, cannot be realised satisfactorily with conventional technologies.
The requirement for extremely small movement steps of a hydraulic consumer occur e.g. in tooling machine feed systems, and in the recent past particularly for solar panels which stepwise have to track the position of the sun as precisely as possible, and at best always after some minutes. As large numbers of solar panels in most cases in solar power plants are controlled by discrete control devices, the above-mentioned complicated structures and the sophisticated control technology result in a cost factor which cannot be tolerated. In addition, such control devices for solar panels contain an electrohydraulic control section which, of course, only should consume a fraction of the electric energy which is gained by the solar panel itself. This calls for extremely weak solenoids operating with a minimum consumption of electric power. However, extremely weak solenoids normally are not apt to control extremely small movements of the hydraulic consumers.
Besides control devices for adjusting solar panels according to the position of the sun and for tooling machine feeding systems there are many further applications where hydraulic consumers sometimes have to be moved in small steps.
An electrohydraulic control device for a working vehicle known from DE-A-10 2005 009 843 comprises a closed hydraulic working circuit for the hydraulic consumer designed as a differential cylinder. A reversible hydraulic pump is the sole pressure source used to extend or retract a piston rod of the hydraulic consumer. A pressure accumulator is provided in a working line between two control valves. The pressure accumulator has two chambers separated by a diaphragm, both chambers having variable volumes for the hydraulic medium and a biasing medium. The control valves are 2/2-way directional valves and are solenoid actuated counter to spring force. The pressure accumulator serves to preliminarily store hydraulic medium under bias pressure, which hydraulic medium stems from the difference from the volumes between the piston rod side chamber and the piston side chamber of the hydraulic differential cylinder. Hydraulic medium stored in the pressure accumulator then is fed into the pump delivery flow when the pump is operating.
A device in a hydraulic/pneumatic combination compound structure is known from FR-A-2700811, and is used to generate rectangular pressure signals in a testing probe connected to a hydraulic part. A series of cam actuated 2/2-way directional valves, which are arranged in series operate such that a leakage-free blocking position can be adjusted preliminarily for each 2/2-way directional valve.